During the past decade, immunohistochemical (IHC) stains have become an integral part of the diagnostic process in surgical pathology. IHC stains are used with conventional histopathological stains, as adjunctive assays. They are critical in correctly diagnosing poorly differentiated malignancies, viral infections, and tumor prognoses, such as in the use of estrogen receptor (ER) analysis for breast carcinoma. Since IHC assays are relatively new in medical practice, simple methods for accurately and reliably monitoring quality control has not yet been developed.
Traditional microscopic analysis of biopsy samples demonstrates overall cellular and tissue architecture. With the commonly used hematoxylin and eosin stain, for example, nuclei are colored purple (with hematoxylin) and the cytoplasm red/pink (with eosin). Often, the type of information available from such stains is insufficient for accurate diagnosis. IHC stains have the ability to extend the level of analysis on a biopsy sample, beyond cellular size and shape, to a molecular level. Tissue samples can be probed for the presence of specific proteins using monoclonal and polyclonal antibodies. The presence of such proteins can be indicative of the cellular lineage of a tumor, facilitating a diagnosis or prognosis with certain types of anti-tumor therapies. Alternatively, the presence of microbiologic agents, such as viruses or bacteria, can be detected using appropriate antibodies.
Quality control is an important aspect of any clinical assay. To assure that clinical test results are accurate, controls should be run with all in vitro diagnostic tests. Quality control in the clinical laboratory is mandated by the Clinical Laboratory Improvement Act of 1988 (CLIA '88). This act outlines the regulatory requirements that a clinical laboratory must meet in order to obtain accreditation; controls and proficiency testing comprise a significant portion of the act. Many tests in the hematology and chemistry sections of the laboratory include controls that are provided by the manufacturer. In contrast, most histology laboratories generate their own controls from excess tissue specimens. With laboratories generating their own control tissues, there is little inter-laboratory standardization. The need for better quality control of immunoreagents was recognized in the late 1980's and led to workshops convened by the Biological Stain Commission to address the issue.
The histology laboratory has lagged behind other sections of the clinical laboratory in the implementation of optimal controls. A negative control is easy to perform. It comprises a serial section of the same tissue with an irrelevant, isotype-matched primary antibody. Standardized positive controls have been harder to achieve. Presently, each laboratory is left to fend for itself in creating, storing, and validation positive tissues controls.
Most histopatholoy laboratories use tissue samples previously documented to contain the particular antigen as positive controls. The laboratory documents, sections, and archives a bank of tissues that will serve as tissue controls for IHC. As the tissue controls are depleted, new tissues/tumor samples are procured to replace those expended. During each daily IHC assay run, each antibody is tested on a positive tissue control. In this manner, each antibody is validated at a specified dilution.
An improvement was described by Battifora (U.S. Pat. Nos. 4,820,504 and 5,610,022), whereby multiple positive control tissue fragments, often-of tumors, are embedded together in a single paraffin block. This “multi-tissue tumor block” simplifies the sectioning process of positive tissue controls. Rather than archiving and sectioning numerous blocks of tissue, the tissue controls are embedded together in a single paraffin block. Therefore, the group of archived tissues can be sectioned simultaneously, with a single stroke of a microtome blade.
A slightly simpler method of preparing multi-tumor tissue paraffin blocks was described by Furmanski et al. (U.S. Pat. No. 4,914,022). The improvement involved embedding tissue cores in a paraffin block. The cores were cut from the tissue of origin with the use of an ordinary plastic drinking straw.
The use of multi-tumor tissue blocks as positive controls does not solve three important problems. One of the most important aspects that a positive control should address is the early detection of reagent failure. The ideal method of detecting early failure is to determine the level of sensitivity at the working concentration of antibody. Sensitivity is determined by titrating the antigen concentration until the antigen is no longer detected. In this manner, the assay can be stated as capable of detecting a certain amount of antigen, e.g., nanomoles or picomoles of antigen. The limit of sensitivity should ideally be checked daily so that trends (towards increasing or decreasing sensitivity) can be detected. It is impossible to perform this type of analysis using tissue sections as controls since there is no practical method for quantitation and titration of antigen in a tissue section.
In addition, tissue sections as controls do not control for performance error. Cutting (with a microtome) and mounting tissue sections on glass slides is labor intensive. Therefore, the aforementioned types of tissue positive controls (such as multi-tumor tissue blocks) are usually tested once per assay run. Because of the associated labor costs, few laboratories place a positive tissue control on each microscope slide. Thus, if there is an error by placing an incorrect antibody (or no antibody) on the sample, it may be impossible to detect. Importantly, the control slide may be correctly treated (verifying the reagent quality) but the sample slide can still be incorrectly treated. The sample would therefore be interpreted as a negative result, although the cause of the negative result is an error in the assay procedure. The present system of positive tissue controls does not control for errors in procedure.
A third problem with tissue sections as positive IHC assay controls is that tumor tissues inherently have a varied, non-standardized amount of antigen. Therefore, tissues do not provide a ready means for calibrating the intensity of the immunologic reaction to an external reference standard. For certain IHC assays, the absence of external reference calibrators is a serious problem. Notably, IHC assays for estrogen receptor and progesterone receptor have become the gold standard for previously quantitative assays that were performed in a test tube. In the absence of such calibrators, staining is. typically quantified as 0-4+ staining intensity, an arbitrary standard that depends upon the reagents, protocol, and time duration of calorimetric development. Because of significant inter-laboratory variability in IHC assay sensitivity, each hospital laboratory must develop its own threshold for determining a positive result. This feature leads to non-standard and sometimes incorrect results. These errors can have therapeutic impact on patient care.
Therefore, a standardized, practical positive tissue control for clinical IHC assays should have the following characteristics to be clinically accepted and scientifically meaningful:
1. Antigen specific. A positive reaction should indicate the presence of only the antigen being assayed.
2. Available in virtually unlimited quantities, so that the positive control has constantly controlled characteristics with the passage of years.
3. Inexpensive. With cost pressures mounting on hospital laboratories, an expensive positive control will most likely not be broadly adopted into routine practice.
4. Stable over a prolonged period of time, ideally without the need for freezing.
5. Standardized, so that each laboratory will have the exact same positive control substrate.
Currently, there is not a quality control reagent or device available for cytochemical procedures that has all of the above characteristics.